The White House has signed a memo allowing troops stationed at the border to take on some law enforcement roles including using lethal force, if necessary. Some experts say the directive is at odds with the Posse Comitatus Act. The federal law, which dates back to the 19th century, forbids active military members from engaging in civilian law enforcement roles. The World’s Carol Hills interviews William C. Banks, a professor of law at Syracuse University, and co-author of “Soldiers on the Home Front: The Domestic Role of the American Military.”